Because in the winter there isn't nearly the levels of IR and UV as there is in the summer, the days are also way shorter. So while it might be slightly cooler in the winter it cannot be compared to the amount of heat blocked in the summer. If it's cold enough to run the heater, your going to have to run the heater anyways. If your talking about a sight chill, where it might be slightly warmer without tint, just use the heated seats?
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Window tint/interior vehicle temperatures in the winter...